A variety of regional standards exist for the properties of diesel fuels. In particular, the requirements for the cold flow properties of diesel fuel can vary based both on a geographical and a seasonal basis.
One method for improving the cold flow properties of a diesel fuel is to subject the diesel fuel to catalytic dewaxing. The goal of the dewaxing process is to improve the cloud and pour point of the diesel fuel while causing a minimum of loss in yield and/or in cetane. By improving the cloud and/or pour point of the feed, a larger portion of the original feed can be used as diesel fuel, which reduces the amount of feed that is diverted to a lower value process, such as FCC cracking for making gasoline. As a result, process improvements that lead to greater cloud point changes at the same severity are desirable, as such improvements enable the use of a broader slate of feeds for diesel production.
One problem for catalytic dewaxing of diesel fuel is that the dewaxing catalysts are typically sensitive to contaminants such as sulfur or nitrogen. This can pose various problems in developing a stable, integrated diesel production process. Typical diesel fuels are often hydrotreated to remove sulfur to a low level, such as 10 wppm or less. Such diesel fuels are often clean enough to be exposed to a dewaxing catalyst without severely harming the catalyst. However, if a process upset occurs and sulfur contaminants remain in the diesel, the catalyst can be poisoned, and some of the loss of activity may be permanent. Additionally, in diesel fuel production processes involving hydrocracking of heavier feeds, substantial amounts of sulfur and/or nitrogen may remain in the hydrocracked feed. These contaminants can be removed by hydrotreatment, but in order to include a dewaxing step, a conventional dewaxing catalyst may have to be moved to a separate reactor.